A bioassay in mice for human cell-mediated immunity to skin-test antigens was developed. A seven-by-seven "checkerboard" testing for cross-reacting tumor rejection antigens between methylcholanthrene-induced squamous cell carcinomas in mice revealed several shared antigens, in contrast to previously reported findings that chemically induced sarcomas in rodents possess tumor-unique, non-cross-reacting antigens. In a mouse model for a cancer skin test, tumor-specific depression of the test by intraperitoneal inoculation of tumor homogenate was shown. Sarcomas were routinely derived from Balb/3T3 cells and C3H 10T1/2 cells by implanting them subcutaneously attached to glass or plastic substrates. The selective growth of tumor cells over normal cells on transparent sheets of Teflon was demonstrated.